Abstract: A setting-type cementitious composition useful for a variety of applications, including, in a preferred embodiment, a joint compound for finishing joints between adjacent gypsum wallboards, comprising a first or cementitious component in pre-mixed form comprising an aqueous slurry of calcium sulfate hemihydrate and a retarder formed of two ingredients, one an organic retarder and the other a retarder which is a chelating agent, and a second or accelerator component comprising an accelerator material preferably in aqueous solution, the first component having an adequately long shelf life suitable for marketing, and, when the cementitious component is mixed with the second component for acceleration, the cementitious mixture acquires excellent application properties maintained throughout a suitable working period. The cementitious component hydrates after a suitable period to form a set material of excellent properties.